Recently, optical fiber, which is applicable to coarse wavelength division multiplexing (hereafter, abbreviated as “CWDM”) transmission, with a lower loss in the band of wavelength 1380 nm (absorption loss due to OH groups), has attracted attention.
The optical fiber with a lower absorption loss due to OH groups enables structuring an inexpensive CWDM transmission system; and in addition, the production cost is also substantially equal to that of typical single mode fiber. Consequently, the optical fiber has a great cost merit, so many companies proceed with research and development and products are commercialized.
When hydrogen diffuses into an optical fiber, it causes increased absorption loss due to OH groups, making so it necessary to prevent hydrogen penetration into the optical fiber. For drawing the bare optical fiber in the manufacturing of the optical fiber, a means to prevent hydrogen penetration into the bare optical fiber is provided.
FIG. 7 is a schematic diagram showing a manufacturing device for an optical fiber used in a conventional manufacturing method for an optical fiber.
In FIG. 7, a reference symbol 31 indicates a drawing furnace. An optical fiber preform 32 is mounted inside the drawing furnace 31 to be axially movable, and a lower end of the optical fiber preform 32 is drawn.
In the manufacturing of the optical fiber, first, optical fiber preform 32 containing as a main component silica-based glass is placed within the drawing furnace 31, and its end is heated to approximately 2000° C. at high temperature in an inert gas atmosphere, such as argon (Ar) or helium (He), and is drawn, thereby obtaining a bare optical fiber 33 with an external diameter of 125 μm.
Subsequently, the bare optical fiber 33 is sent to a mechanism to slowly cool the optical fiber, such as an annealing furnace 34 (hereafter, referred to as “annealing mechanism”), the cooling speed of the bare optical fiber 33 is changed and the optical fiber is slowly cooled.
The bare optical fiber 33 drawn out to the outside of the annealing furnace 34 is cooled to a temperature suitable to the formation of a coating layer for a next process. In the cooling process, it is naturally cooled in an atmosphere surrounding the bare optical fiber 34 or is forcibly cooled by supplying cooling gas, such as helium or nitrogen gas, using a cooling cylinder 35.
The bare optical fiber 33 cooled in the cooling process is coated with a coating layer made of ultraviolet ray curable resin, and which is made of a primary coating layer and a secondary coating layer, by a resin coating device 36 and a UV lamp 37 for the purpose of protecting the bare optical fiber 33, thereby obtaining an optical fiber 38 with an external diameter of 250 μm.
In addition, the optical fiber 38 is turned to another direction by a turning pulley 39, and is wound onto a winding drum 42 via a drawer 40 and a dancer roller 41.
Furthermore, a method for providing a coating layer onto the bare optical fiber 33 is not only a method where after a resin for the primary coating layer formation and a resin for the secondary coating layer are applied by a single resin applicator 36, the resins are cured by a single UV lamp 37, as shown in FIG. 7, but with a method where after a resin for primary coating layer formation and a resin for secondary coating layer are applied by two different resin applicators, the resins are cured by a single UV lamp, and another method where after a resin for primary coating layer formation is applied by a first resin applicator, the resin is cured by a first UV lamp, and after a resin for the secondary coating layer is applied by a second resin applicator, a resin cured by a second UV lamp can also be used.
In the conventional manufacturing method for an optical fiber, in order to reduce Rayleigh scattering and to reduce loss at a wavelength of 1550 nm (for example, refer to Patent Documents 1 to 3), or in order to control the increase in absorption loss caused by OH groups, it tends to slow the cooling speed and to cool the bare optical fiber 33 drawn out to the outside of the drawing furnace 31, by adjusting the drawing speed in a temperature region corresponding to the purpose, respectively, or by prolonging the annealing time.
As described above, when annealing is performed in the drawing the bare optical fiber 33, residual OH groups in the optical fiber preform 32 diffuse and hydrogen is thermally dissociated from the OH groups. In addition, diffusion of the dissociated hydrogen increases. Increased absorption loss due to the OH groups or the combination of a non-bridging oxygen hole center (hereafter, abbreviated as “NBOHC”) in the optical fiber and hydrogen results in increased absorption loss due to the OH groups.
Means to resolve the problem are proposed, for example, in Patent Documents 4 to 6.
In Patent Document 4, an optical fiber preform having a substrate tube, a cladding layer inside the substrate tube and a core layer inside the cladding layer, and a barrier layer established between the substrate tube and the cladding layer, and a manufacturing method for an optical fiber using the optical fiber preform are proposed. The barrier layer is formed by depositing a substance with a low OH diffusion coefficient between the substrate tube and the cladding layer, and the penetration of the residual OH groups within the substrate tube to the cladding layer is prevented.
In Patent Document 5, a manufacturing method for an optical fiber is proposed where a first cladding with an external diameter “D” is deposited so as to surround a core with an external diameter “d” using a vapor-phase axial deposition method; a porous core rod satisfying a relational expression, D/d≧4.0, is formed; the porous core rod is dehydrated and the OH group concentration is reduced to 0.8 wtppb or less and vitrified to form a core rod; the transparent core rod is heated and elongated; a second core rod is deposited surrounding the core rod after elongating using the vapor-phase deposition method; the second clad is dehydrated so as to reduce the OH group concentration to 50 wtppm or less; it is vitrified to form an optical fiber preform; and after the optical fiber preform is drawn, it is maintained in a heavy hydrogen atmosphere for a pre-determined time.
In Patent Document 6, in a manufacturing method for an optical fiber where raw material gas is reacted and a glass fine particle aggregate is obtained, and the glass fine particle aggregate is sintered to vitrify it, a method via a first heating process to pre-dehydrate the glass fine particle aggregate and next, a second heating process to increase the temperature to a vitrification temperature, within the temperature range of 950 to 1,250° C. where the glass fine particle aggregate is not remarkably contracted, substantially in the oxygen gas atmosphere containing 1 mol % to 20 mol % of chlorine or chlorine compound, is proposed.
The manufacturing methods for optical fibers proposed in Patent Documents 4 to 6 have a problem where an absorption loss due to OH groups increases depending upon drawing conditions for the optical fiber. Further, there is another problem where a production cost increases.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-338289
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-321936
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2000-335933
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2002-535238
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2002-187733
Patent Document 6: Japanese Patent No. 2,549,615